Enhanced Cr(VI) adsorption by DOM-coated iron-manganese binary oxides: Process and mechanisms

Fe-Mn binary oxides (FeMnO x ) show promise for Cr(VI) remediation, but their interaction with dissolved organic matter (DOM) remains poorly understood. Here, we developed a DOM-coated Fe-Mn binary oxide (FeMnO x -DOM) that achieved 72 % higher Cr(VI) removal compared to uncoated FeMnO x under optimal conditions. Kinetic and isothermal analyses revealed that Cr(VI) adsorption on both FeMnO x and FeMnO x -DOM follwed spontaneous endothermic monolayer chemisorption . The reaction rate was dominantly governed by chemical reaction, and DOM functionalization amplified the regulation through enhanced surface reactivity. DOM coating increased the specific surface area from 55.388 m²/g (FeMnO x ) to 223.609 m²/g (FeMnO x -DOM) while shifting the isoelectric point from 3.87 to 7.56, indicating surface charge reversal. X-ray photoelectron spectroscopy quantified a 49.6 % increase in reductively active Mn(II/III) species (59.48 %→89.04 %), correlating with a marked decrease in post-adsorption Cr(VI)/Cr(III) ratios from 11.79 to 1.70. The phenolic O-H groups in DOM, and Fe-OH, Fe-O, Mn-O bonds in FeMnO x were involved in Cr(VI) adsorption. DOM, as both an electron transporter and a ligand provider, accentuated Cr(VI) adsorption by reinforcing the efficacy of redox and surficial complexation. This study can provide references for designing adsorbents with high efficiency adsorption synchronous detoxification of Cr(VI). • DOM-coated FeMnO x enhanced Cr(VI) adsorption by up to 72 %. • Phenolic O-H groups in DOM cooperated with Fe-OH/Fe-O/Mn-O bonds in FeMnO x . • DOM mediated the redox of Fe and Mn, generating more reductive Mn. • Cr(VI)/Cr(III) in FeMnO x -Cr and FeMnO x -DOM-Cr were 11.79 and 1.70, respectively. • The redox and surface complexation mechanisms of Cr(VI) adsorption were reinforced.